Cardiac rupture and aneurysm formation following acute myocardial infarction may be related to degradation of structurally important collagen molecules by inflammatory cell proteolytic enzymes. Using a rat model of acute myocardial infarction, significant collagen degradation was found to occur in the first twenty-four hours following experimental infarction. This degradation was associated with a marked increase in proteolysis in the infarct region. This collagen degradation was inhibited by eliminating inflammatory cells by prior whle body irradiation. Electron microscopic studies demonstrated collagen breakdown in control infarcts and preservation in infarcts of irradiated, leukopenic animals confirming an important role of white cell proteases in early degradation of collagen after acute myocardial infarction.